METAMORPHOSIS OF HOLOTHURIANS. 215 
metamorphosis being less marked—i.e., growth is more 
continuous, as in the Crinoids. 
In Holothuria tremula and Synaptula vivipara there has 
oeen observed a very slight metamorphosis, the young de- 
veloping directly in a marsupium, as in the star-fishes and 
sea-urchins. Cladodactyla crocea Lesson, of the Falkland 
Islands, according to Sir Wyville-Thompson, carries its 
young in a sort of nursery, being ‘‘ closely packed in two con- 
tinuous fringes adhering to the water-feet of the dorsal am- 
bulacra.’’ He also found that in Psolus ephippifer Wyville- 
Thompson, which is covered with calcareous plates, there is 
a dorsal group of larger tessellated plates, each supported 
by a broad pedicel embedded in the skin. Under these 
mushroom-like plates brood-cavities or cloister-like spaces 
are left between the supporting columns, and in this archi- 
tectural marsupium the embryos directly develop into sea- 
cucumbers. It follows that in all free-swimming Echino- 
derm larvee, there is a true metamorphosis as distinct as in 
the butterfly, while in other forms in which development is 
direct the embryo is sedentary and lacks the cilia and vari- 
ous appendages so characteristic of the ordinary larval 
Echinoderms ; thus there are different stages in the differ- 
ent classes of Echinoderms between direct development ot 
continuous growth, and a complete metamorphosis like that 
of the star-fish or sea-urchin, in which the pluteus or larva 
is but a temporary scaffolding, as it were, for the building 
up of the body of the adult. 
Turning now to the classification of the Holothnrians, 
and beginning with the lowest, simplest, most generalized 
forms (which are also remarkably worm-like), and ascend- 
ing to higher or more complicated forms, we find that there 
are two orders, those without feet (Apoda) and those with 
ambulacral feet (Peduta).* 
* Tt is possible that the Holothurians shouid he divided into two sub- 
classes, one Diplostomidea Semper, in which the body is spherical and 
the mouth and anus are close together, with ten ambulacral rows, etc., 
and the normal, cylindrical, bipolar Holothurians. Semper’s Diplostomi- 
dea is based on Rhopalodina lageniformis Gray, from the Congo Coast, 
and regarded hy Semper as the type of a fifth class of Echinoderms. 
